1. Field
This invention relates generally to Transmission Electron Microscope (TEM) and, more particularly, to Scanning Transmission Electronic Microscope (STEM).
2. Related Art
Transmission electron microscope (TEM) is a system which transmits a beam of electrons through a very thin specimen. An image of the area of the specimen illuminated by the beam is formed from the electrons exiting the other side of the specimen. A scanning transmission microscope (STEM) is a system wherein the electron beam is focused to a point and is scanned over a selected area of the specimen. TEM systems having the appropriate additional parts may operate in either TEM or STEM modes; however, dedicated STEM systems are also available.
FIG. 1A is a simplified sectional illustration of a conventional TEM system 100, that may operate in either TEM or STEM mode. As seen in FIG. 1A, a conventional TEM/STEM system 100 comprises an electron beam column 102 within a vacuum chamber 104. The electron beam column 102 emits an electron beam 106, which is focused using conventional electron optics (not shown) for scanning a sample 110 during the scanning mode of the TEM/STEM system 100. A relatively large amount of the electrons pass directly through the sample 110. Some electrons 114 are scattered by the sample 110 following impingement of the electron beam 106 thereon. It is known in the art that the number of scattered electrons and the distribution of the scattering angle θ are related to the atomic number Z of a scanned atom within the sample 110 and to the thickness of the sample. As seen in FIG. 1A, a relatively small number of scattered electrons with a relatively small scattering angle θ1 of scattered electrons 116 indicate that the atomic number of the scanned atom is relatively small and/or that the sample is thin at that scanning location. Similarly, a relatively large number of scattered electrons with a relatively large scattering angle θ2 of scattered electrons 118 indicate that the atomic number of the scanned atom is relatively large and/or that the sample is thick at that scanning location. Thus, information regarding the composition of the atomic numbers of the atoms in the scanned sample may be determined by the number of electrons that undergo scattering and the distribution of the scattering angles. However, accurate determination is sometimes difficult due to the signal contribution from the thickness of the sample.
A standard Bright Field detector 120 may be used to detect the un-scattered electrons passing through the sample 110 or detect electrons with a relatively low scattering angle. The Bright Field detector 120 is typically formed of a silicon-diode detector, suitable mainly for detecting un-scattered electrons or scattered electrons with a relatively small scattering angle. The signal obtained from the bright field detector is used to generate an image of the sample which conveys the physical structure of the sample, but not the atomic composition of the sample.
One or two annular detectors are usually provided for detecting scattered electrons in the STEM mode of operation. An Annular Dark Field (ADF) detector 121 for smaller scattering angle electrons and a High Angle Annular Dark Field (HAADF) detector 130 for a range of larger scattering angle electrons may be provided. The dark field detectors can be used to obtain information about the atomic composition of the sample. The ADF detector for smaller angles and relatively high number of scattered electrons may be a silicon diode detector or a scintillator based detector. The silicon diode detector performance is limited by the relatively high dark current noise and low amplification. Thus it is suitable for a relatively high signal operation mode where the total current impinging on the silicon diode is higher than 10 pA (Pico-Ampere). The ADF detector 121 may be an annular detector provided about the bright field detector 120, as shown in the example of FIG. 1A.
The HAADF is typically a scintillator based detector, which is suitable for detecting scattered electrons emitted from the sample 110, wherein the electron beam current is not high. As seen in FIG. 1A, a HAADF detector assembly comprising a scintillator based detector assembly 130 is provided for detecting the scattered electrons emitted from the sample 110. The scintillator based detector assembly 130 comprises a scintillating surface 134 formed of a scintillating material, such as YAP, YAG, or a layer of phosphorous scintillating material such as P47 or R42, for example. The scintillating surface 134 is formed with an aperture 140 for allowing the un-scattered electrons to pass through to the Bright Field detector 120 and the ADF detector 121 if used. Upon impingement of an electron 114 on the scintillating surface 134, a light signal 142 is formed and guided by a light guide 144 to a Photomultiplier Tube (PMT) 150 and impinges thereon. The scintillator based detector assembly 130 may be coupled to a retracting mechanism 184, which is provided to retract the scintillator based detector assembly 130 from the electron beam path wherein the scintillator based detector assembly 130 is not in operation, such as during a TEM detection mode.
Turning to FIG. 1B, which is a simplified sectional illustration taken along lines IB-IB in FIG. 1A, a top view of a BF detector 120, ADF detector 121, and HAADF scintillator surface 130 are shown. Electrons 116 and 118 illustrate different electrons that are scattered from the sample at different scattering angles θ1 and θ2, respectively. The signal generated by these electrons is detected by the PMT 150. Since a photon generated from any location on the scintillating surface 134 is transferred to the PMT 150 by the light guide 144, only intensity information is obtained. That is, for every scan pixel of the beam in STEM mode, one pixel data is obtained from the PMT, indicating total intensity from the scintillator. However the spatial information comprising the electron impingement location on the scintillator is lost. Thus, some of the sample material associated information is lost as well.
Moreover, in order to obtain better scattering angle resolution, the optics or the position of the detector needs to be adjusted. For example, the system can be set to be sensitive to a specific angle beforehand, and a scan is then performed. Then the system setup is thereafter changed to be sensitive to another angle, and another scan is performed. In this manner, information relating to specifically selected scattering angles can be obtained and correlated to the atomic composition of the sample. However, it should be appreciated that: i. the specific angle for each scan must be selected beforehand, and ii. a registration procedure must be performed to align the signals from all of the scans. This procedure is tedious, slows the analysis, and may miss important information if the wrong angles are selected. Moreover, some samples get destroyed by the electron beam, enabling only a single scan. To perform a scan for a different angle, another sample must be prepared.
It has also been proposed to use a circular area detector to obtain BF, ADF, and HAADF images simultaneously. The area detector is formed by 16 detectors, each lined via a fiber cable to its dedicated PMT. The signals from the PMT are digitized and displayed on a computer monitor. This arrangement basically replaces the standard, three detectors, BF, ADF and HAADF, arrangement.
In view of the above, a STEM arrangement that enables resolution of the scattering angles without the need for repeated imaging and registration would be beneficial. A STEM arrangement that enables selection of scattering angles after the scan would also be beneficial. Furthermore, a STEM arrangement that enables resolution at several scattering angles simultaneously and using only a single scan would also be beneficial. Furthermore, a STEM arrangement that enables resolution of the scattering angles without disrupting the standard BF detector would also be beneficial.